Polaris (star)

Polaris is the name of the star α Ursae Minoris (α UMi, 1 UMi), the brightest star in the constellation Ursa Minor, with apparent magnitude + 1.97, which makes it visible to the naked eye at night. It is also called Polar Star or North Star, due to its proximity to the north celestial pole..

As the closest Cepheid variable, its distance is used as part of the cosmic distance ladder. The stellar parallax obtained by Hipparcos gives a distance to Polaris of approximately 433 light-years (133 parsecs), while the successor mission Gaia gives a distance of approximately 447.6 light-years (137.2 parsecs).). Calculations by other methods vary widely.

Though it appears to the naked eye as a single point of light, Polaris is a triple star system, made up of the primary, a yellow supergiant named Polaris Aa, in orbit with a smaller companion, Polaris Ab; the pair is in a wider orbit with Polaris B. The outer pair AB was discovered in August 1779 by William Herschel.

Etymology

The name Polaris comes from the Latin Stella Polaris, literally 'polar star'. Given its proximity to the north pole, the star has received great attention since ancient times; Greek navigators called it Kynosoura or Cynosura, meaning 'Dog's Tail'.

In China it was known by various names: Pih Keih, Ta Shin and Tien Hwang Ta ti, 'the Great Ruler' Heaven's Imperial. In northern India she received the title of Grahadhara , "Support of the planets". She represented Dhruvá and the Persian astronomer Al-Biruni pointed out that in his time—around 1000 B.C. C.—represented Dhruvá himself. It was an object of worship, as it has been until recently for the Mandeans of the Tigris and lower Euphrates. In Damascus, he received the title of Mismar, 'the Needle,' or 'the Nail'.

Pole Star

Due to the movements of the Earth's axis of the precession of the equinoxes and nutation, the stars do not really remain fixed in the celestial vault. Thus Polaris will not always be the closest star to the North Pole, any more than it was in the past. Polaris continues to approach our north celestial pole, to then gradually move away from it, and to which it will return within a complete precession cycle, in 25,780 years. Other stars, including Thuban (α Draconis) and Vega (α Lyrae), were the North Star in the past and will be again in the future.

To locate Polaris in the sky, simply extend the line determined by the stars Merak and Dubhe (β and α Ursae Majoris, respectively), finding the Pole Star at a distance equivalent to five times the separation distance of the two aforementioned stars.

The equivalent of Polaris at the south pole is the star σ Octantis, thus called Polaris Australis. However, being a fifth magnitude star barely visible to the naked eye, in practice the constellation of the Southern Cross is used to locate the south celestial pole.

Physical characteristics

Polaris is a yellow supergiant of the spectral type F7Ib-II located 431 light years from Earth with a luminosity 2440 times greater than the Sun. Its radius is 45 times greater than the solar radius. It is a Cepheid pulsating variable whose brightness fluctuates between magnitude +1.86 and +2.13 in a period of 3.9696 days; currently (2010) its pulsations have stopped or are so low that they cannot be detected. In 2018 the Gaia satellite has finally determined its parallax and hence its distance: 447 light years.

Polaris forms a star system with two companion stars. The brightest of them, Polaris B, is of the eighth magnitude and is located at 18 seconds of arc; is a main sequence star of type F3V that is at least 2400 AU from the main star Polaris A. Closer to the latter is the other companion, Polaris C, initially discovered by spectroscopy. and later resolved by the Hubble Space Telescope. It is a yellow dwarf of type F7V that moves in an eccentric orbit at an average distance of 17 AU with respect to Polaris A and whose orbital period is 29.6 years.

Polaris, The Beatles

On February 5, 2006, at 00:00 UTC, NASA broadcast The Beatles' (John Lennon) song "Across the Universe" in the direction of the star Polaris. The transmission was made using a 70 m antenna at the Madrid Deep Space Communications Complex, located on the outskirts of Madrid, Spain. And it was done with an “X band” transmitter, which gave the antenna 18 kW.

The goal was to celebrate the 40th anniversary of the song, the 45th anniversary of the Deep Space Network (DSN) and the 50th anniversary of NASA. The idea was conceived by Beatles historian Martin Lewis, who invited all Beatles fans to play the song as if it were going to be sent to a distant star. This is the second time a piece of music has been intentionally transmitted into outer space (the first being the interstellar message: "1^st Theremin Concert to Aliens"), and it was endorsed by Paul McCartney, Yoko Ono and Apple Records.